Gravity Tractor Doh

Due to the lasting nature of communicating on these various internet sites, I am on record in several places as saying that gravity tractors for moving asteroids are stupid. I could see no way that gravitational attraction between an asteroid of a few hundred thousand tons and a spacecraft of a few thousand pounds could make sense as a means of linking them for thrust, however gentle. So after my last snark over at transterrestrialmusings.com, Doug Jones destroyed my little conceit with numbers. John Schilling weighed in with a bit more conceit destruction.

Doug brought in something that I hadn’t realized with his numbers. He worked them with a spacecraft mass a couple of hundred tons. I had made the assumption that the craft would be yet another of the fragile constructs similar to most probes flying out there now. With a couple of hundred tons of spacecraft though, it becomes possible to have enough coupling to actually move the asteroid, albeit very slowly. The other two things Doug and John Shilling brought in were the difficulty of finding good attachment and the fragile nature of rubble pile asteroids. The lasso and drag that I had assumed could be more difficult than I had thought and might just tear the rubble pile apart.

Without the numbers from a known rocket scientist, plumber, I might have quibbled a bit more even on the losing end. I am not, after all a graceful looser, nor do I like being shown up as a fool. So as revenge, I modify the concept to fool people into thinking that maybe it was my idea all along. If two hundred tons is good, two thousand tons would be better, especially with ISRU.

Sending a two hundred ton vehicle to a dangerous asteroid would be a bit difficult today. IMLEO would probably be on the order of a thousand tons or so. Even under the dinokiller or city buster threat, the empire builders might just delay things by scarfing funding for heavy lift, nuclear thermal, or other program that could easily extend beyond the launch window for the threat.

I suggest it might be possible to get similar results with less spacecraft with a bit of low tech ISRU. The spacecraft uses the previous flyby surveys to locate a boulder that masses enough to generate the gravity attraction desired. It lassos the boulder from the rubble pile and pulls it free of the rest of the asteroid. The spacecraft then pulls the boulder into the appropriate location to attract the rest of the asteroid along the desired thrust vector. The craft lets out enough tether to hold the boulder in the right place while being well clear of the asteroid itself.

The advantages would be less cosine losses from the thrusters as the spacecraft itself would be well clear of the asteroid at the end of the lasso/tether. Acceleration could be higher with more mass of ISRU boulder than deliverable spacecraft. And less spacecraft to get the job done, which could be the difference between on time success and too little too late with the original concept if too much time is wasted on ‘perfection’.

Why bother with tractors at all? What about the idea of attaching a solar sail to the asteroid? If it’s a rubble pile, a carbon-fiber net could be deployed to keep it cohesive. Once that’s secure, or if the object is determined to be cohesive already, a component containing the sail can attach on the dark side of the object and deploy the sail. The sail can be sized to account for the mass and speed of the object, which will then be slowed and moved out of its trajectory. A light sail and carbon-fiber net might be cost-effective enough to launch from Earth’s gravity well. This is not a new idea, but I wonder why it’s not suggested as much in lieu of the gravity tractor concept?

Guthrie,
I bothered because I made a few boneheaded knee jerk statements about how stupid and worthless gravity tractors were and was corrected.
1.The gravity tractor attracts all pieces of the asteroid at the same time which doesn’t have to worry about cohesion.
2. The asteroid may not cooperate with attaching tethers or nets due to tumble, irregular shape, or other factors that didn’t happen to be foreseen before the mission was launched.
3. I think your solar sail might well be superior to the ion drives suggested by several sources.
4. We are all still brainstorming possibilities and the jury should remain out on any specific tech until it is time to start making firm decisions.

Andrew,
I can’t see dragging a rock there from somewhere else as that would seem to defeat the purpose of local ISRU. If there is no big rock then go to original plan of two hundred ton spacecraft. Or devise a tech for gathering sandstone in a bag.

It seems like pulling a boulder out of a rubble pile asteroid would suffer the same difficulties of attachment and rubble pile rearrangement that you would have if you tried to lasso or bag the whole thing.

Bob,
It would depend on the composition of the asteroid as determined by a preliminary survey mission. If the asteroid has manageable tumble characteristics, then bagging the whole thing would be clearly superior. If the asteroid has unmanageable qualities, and a loose boulder of the right size, then it could be easier to grab just the boulder.

I can picture a preliminary survey mission with a few impact anchors, blasting charges, and light nets. A boulder that seems to be right could be pushed with a tiny charge on the order of a pound of explosives. If it is seen to move freely, then an impact anchor could be smacked into a good location. If the anchor takes and doesn’t fracture the boulder, then the main mission just snags the boulder with a hook on a tether. It may take several tries to get a cooperative boulder ready to harvest. Alternately a blast might blow a mix of material clear to be bagged in (asteroid) sub-orbit before it settles to wait for the main mission.

I am not 100% sold on the gravity tractor being the solution to moving an asteroid. After being corrected on the idea though, I am looking for techniques to make it a more useful addition to the tool box. I think there are a dozen or more better methods that might work. The problem is in the might. It might be unfortunate to have the best tools available, only to find out that the asteroid is SAE and all we have is metric.

I think you are going too high tech on this one. Instead of locating and explosively mining boulders, how about this: Shoot a spike into the surface, rendezvous with the end of the spike, and dock your spacecraft to the asteroid with a manipulator arm. Use a scoop at the end of the arm to fill a mylar bag with the maximum mass of regolith that your spacecraft drive can lift above the surface gravity of this particular asteroid. Hover above the surface under power, bag suspended “below”, in whichever orientation you want to move the asteroid.
Additional tricks:
I’d use the bag full of regolith as a heat sink to derive power opposite a nuclear reactor or solar focus. Replace the hot regolith with a fresh supply, or rotate between multiple bags every so often.
If you want to apply high tech back into the scheme, it might make sense to look at a “no-touch” ion thruster like E-IMPAcT from those Elwing guys. Eventually you’ll want to use regolith as reaction mass.

I don’t want to apply high tech for the sake of high tech. I want as many options as possible if we need them. A serious study might well choose your technique for one danger, mine for another, and that of Andrew for a third threat. I want us to win against the universe, not each other.

Unless we have something like 10 years warning we will only have time to send one probe to the cosmic item. It could be solid iron or sand or anything in between. Solid iron will resist a spike where as a spike will straight through sand.

Andrew,
I suppose it would take a lot of thought to design an anchor or anchors that could handle all the possible asteroid densities. I confess I was assuming the reason to do something like this is to move an asteroid for exploitation rather than deflection from earth.
If we ever see an asteroid that is going to hit us in time to do anything about it, I suspect the solution will be to smack it with the fastest and massiest fleet of Orions we can deliver and pray it for the best.

Thanx John. I agree, we want all kinds of options available to us, not try to pick a single ‘winner’ which may or may not work in some circumstances. And here’s yet another reason to return to the moon… we can set up an observation site on the far side dedicated to finding dangerous NEO’s.

Remember that gravity is the tether or hitch. The tractor is a rocket, or perhaps a solar sail.

Bombardelli and Pela’ez have a competing technology in the Ion Beam Shepherd. Exhaust from the shepherd impinges on the rock (or rubble pile), transferring momentum. A second rocket is pointed at 180 degrees to hold the shepherd in position. See